In a dot matrix display apparatus, the positions of pixels are immovable. Therefore, when display data for a dot matrix display apparatus having low resolution, that is, in which the number of pixels is small is to be displayed on a dot matrix display apparatus having high resolution, that is, in which the number of pixels is large, if the display data is not expanded, the display data is displayed on only part of the display area of the high-resolution dot matrix display apparatus and it is difficult to see the contents to be displayed. An actual example of such a case is the case where display data for the dot matrix display apparatus of 640 dots.times.480 lines, that is, the dot matrix display apparatus, in which one display line comprises 640 dots and the number of display lines is 480, is to be displayed on the dot matrix display apparatus of 1024 dots.times.768 lines. In this example, it is desirable that the display data for the dot matrix display apparatus of 640.times.480 should be expanded for the dot matrix display apparatus having 1024.times.768 or nearly 1024.times.768.
It is well-known that an image is obtained which has little visual difference if display data for a low-resolution display apparatus is expanded while the similarity between the brightness distribution of a high-resolution display screen after expansion and that of a low-resolution display screen before expansion is maintained. It is also well-known that the brightness of each pixel after expansion should be an intermediate value of the brightness of peripheral pixels of the corresponding positions before expansion so as to maintain the similarity of the brightness distribution and expand the display data. While there are several well-known methods for calculating an intermediate value, a method which is considered the most common of them is described in the following.
As shown in FIG. 47, when a low-resolution display screen and a high-resolution display screen with screens are of the same size superposed, pixels of low resolution and those of high resolution are slightly shifted with respect to each other. This shift is periodically repeated. As shown in FIG. 48, when attention is paid to one of the pixels of high resolution, it is obvious that the pixel of high resolution stretches over four pixels of low resolution. If it is assumed that the brightness of the pixel of high resolution is H, the brightness of the four pixels of low resolution are L0, L1, L2, and L3, and areas where the pixel of high resolution overlaps each of the four pixels of low resolution are S0, S1, S2, and S3, then the brightness (H) of the pixels of low resolution is calculated by the expression: EQU H=(S0 L0+S1 L1+S2 L2+S3 L3)/(S0+S1+S2+S3).
The brightness (H) of the pixel of high resolution is obtained by calculating the weighted mean of the brightness of the overlapping pixels of low resolution (L0, L1, L2, and L3) by the overlapping areas (S0, S1, S2, and S3). However, the operation for calculating an intermediate value may be based on not only the area ratio, but also a distance between centers of pixels, the square ratio of the distance and the like.
Display data may also be conventionally expanded using a software technique. In this case, display data for low resolution must be read from a memory in an information processing system, the read display data must be converted to display data for high resolution, and conversion results must be written into the memory. Therefore, this method requires a long time for processing, and for example, if the display data for low resolution successively varies, it is difficult to display the display data for high resolution corresponding to the variations.
Further, dedicated hardware for data expansion may be provided in an information processing system, and after display data is expanded in the information processing system using the hardware, the expanded display data may be sent to a dot matrix display apparatus. In this case, unless the expanded display data is sent out from said dedicated hardware at a relatively higher speed than the velocity at which display data for low resolution is sent to said dedicated hardware, the display data for high resolution cannot be displayed according to variations in the display data for low resolution. For example, if display data is also expanded for a display screen wherein resolution is expanded by a factor of 1.5, the expanded display data must be sent out using a clock having a frequency which is 1.5 times as high as the frequency of a clock used to read the original display data. Therefore, in addition to a clock for reading the display data for low resolution, a clock having a higher frequency must be provided for sending out the display data for high resolution, which makes the overall circuit configuration very complicated.
As described above, in the conventional methods for expanding display data, that is, whether software or dedicated hardware is used, the display data is first expanded in an information processing system, and then the expanded display data is sent to a dot matrix display apparatus, which causes problems in that the speed of processing is low and various clocks of different frequencies are needed.